1. Field of the Invention
The present invention relates to an optical pickup device including a laser light source, a light receiving portion, objective lens and the like for recording/replaying information to/from an optical disc recording face in an optical disc drive unit.
2. Description of the Related Art
The general objective lens actuating means provided on the optical pickup device of the optical disc recording/replaying apparatus for CD, DVD or the like comprises a movable lens holder equipped with an objective lens, a lens holder supporting member, a focusing coil and a tracking coil fixed to the lens holder, and yoke and permanent magnet disposed opposing these coils. When drive current is applied to the focusing coil, electromagnetic force is generated by action between the current and magnetic flux from the permanent magnet, so that the lens holder is driven in the focusing direction in which it approaches or leaves the optical disc surface. Likewise, when drive current is applied to the tracking coil, electromagnetic force is generated by action between the current and magnetic flux from the permanent magnet, so that the lens holder is driven in the tracking direction which is a radial direction of the optical disk.
An example of the objective lens actuating means including such a structure is described in Patent Document 1. In the objective lens actuating means described in Patent Document 1, a pair of rectangular flat coils are mounted on the side face of a lens holder with a slight gap therebetween and another coil is mounted above the center of the gap between the pair of the coils. At this time, first, the pair of the coils are mounted on the side face of the lens holder and another coil is mounted such that it overlaps part of the pair of the coils. Further, there has been indicated an example in which the three rectangular flat coils are disposed without any overlapping by reducing the dimensions of those coils.
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-326885 (see FIGS. 6, 8, 1, 3, 6)
However, in the structure of the objective lens actuating means described in Japanese Patent Application Laid-Open No. 2004-326885 in which the three rectangular flat coils are mounted such that they overlaps partially each other vertically, the magnetic flux density applied to these coils drops because the first and second coils mounted tightly to the lens holder side face are farther from the third coil, so that the drive force drops, which is a problem to be solved.
Although to avoid such a problem, the dimensions of the three rectangular flat coils are reduced slightly so as not to produce overlapping of these coils in order to bring them close to the magnet equally, no sufficient drive force is secured because the length of each coil is shortened, so that demand for accelerating recording/replay cannot be met, which is another problem to be solved.
Although the magnetic flux density generated from the magnet is substantially equal in the center of the magnet, the magnetic flux density decreases around the magnet. If part of the coil is disposed in the surrounding area of the magnet in which the magnetic flux density drops, the magnitude of the drive force generated in the coil changes when the coil is disposed in the focusing direction or tracking direction, so that rolling moment is generated with respect to the center of the coil. This rolling moment induces an inclination of the objective lens thereby affecting the recording/replay of the optical disc badly.
This phenomenon will be described with reference to an accompanying drawing. FIG. 9 is a diagram showing an arrangement of coils on the lens holder side face and a magnet opposing them in a conventional optical pickup device. In FIG. 9, a pair of first coils 23, a second coil 4 and a permanent magnet 11 are disposed with the positional relation as in FIG. 9 of Japanese Patent Application Laid-Open No. 2004-326885. Because the pair of the first coils 23 are disposed such that the second coil 4 is sandwiched by them, it comes that both the coils 23 are disposed outwardly in the tracking direction (y direction). This indicates the neutral condition in which the coil is not displaced in the focusing direction or the tracking direction.
FIG. 10 shows a condition in which such conventional lens holder is displaced in the tracking direction from a neutral condition. Because the coil line portions 23a, 23b of the first coil 23 approaches from the peripheral portion to the central portion of magnetic pole area of the permanent magnetic pole 11, drive forces Fa′, Fb′ generated in the coil line portions 23a, 23b increase. On the other hand, the coil line portions 23c, 23d of the first coil 23 leaves from the central portion of the magnetic pole area of the permanent magnetic pole 11 further and consequently, drive forces Fc′, Fd′ generated in the coil line portions 23c, 23d decrease. Thus, a difference is generated between the drive forces in the focusing direction (z direction) generated in the right and left sides of the first coil 23 in pair, so that rolling moment around the x-axis is generated. The lens holder is inclined by this rolling moment.
To reduce generation of the rolling moment in the conventional lens holder shown in FIG. 9, there is no other way than keeping the first coil 23 in an area in which the magnetic flux density is equal by reducing the dimension of the first coil 23. However, no sufficient drive force can be obtained if the dimension of the coil is reduced as described above. Although the dimension of the permanent magnet 11 needs to be increased in order to keep the first coil 23 in the area in which the magnetic flux density is equal with the dimensions of the first coil 23 not changed, the optical pickup device itself cannot help getting bigger.